The invention relates to a cryostat configuration for keeping cryogenic fluids in at least one cryocontainer which is suspended from thermally insulating suspension tubes and/or suspension devices which are connected to an outer jacket of the cryostat configuration, wherein the at least one cryocontainer is subjected to thermal effects and is centered relative to a surrounding container, preferably the outer jacket, using at least three centering elements which are distributed about the periphery of the cryocontainer, wherein one end of each centering element abuts or is mounted to a surrounding container.
A cryostat configuration of this type for a superconducting magnet system of a nuclear magnetic resonance (NMR) measuring means is disclosed e.g. in GB 2 015 716 A and in the document “Superconducting NMR Magnet Design” (Concepts in Magnetic Resonance, 1993, 6, 255-273). The superconducting magnet coil system is disposed in a first cryocontainer with cryogenic liquid, usually liquid helium, which is surrounded by radiation shields, superinsulating sheets and one further optional cryocontainer having cryogenic liquid, usually liquid nitrogen. The liquid containers, radiation shields and superinsulating sheets are housed in an external container which delimits a vacuum space (outer vacuum shell, outer jacket). The superconducting magnet is cooled and kept at a constant temperature by the evaporating helium surrounding it. The elements surrounding the helium container thermally insulate the helium container to minimize the heat input into the helium container and minimize the evaporation rate of the helium.
Magnet systems for high-resolution NMR spectroscopy are generally so-called vertical systems, wherein the opening for receiving the NMR sample to be examined and the homogeneous region of the magnetic field lines extend in a vertical direction. Each of the nested cryocontainers and the outer jacket have an inner tube. The helium container is usually connected to the outer vacuum shell via at least two thin-walled suspension tubes. The cryocontainer is thereby mechanically fixed and the suspension tubes provide access to the magnet, as is required e.g. for cooling and charging. To prevent contact between different cryocontainers at different temperature levels (and thereby prevent heat bridges and increased cryogenic loss), the cryocontainers are radially supported by centering elements which are pressure or tension-loaded. These comprise thin bars of maximum possible length made from a material having poor thermally conducting properties and are e.g. rigidly connected to the helium container but contact the nitrogen container only at points. In this case, the nitrogen container is again connected to the outer vacuum shell via similar centering elements and is thereby centered. Pressure-loaded centerings are conventionally used for smaller rotationally symmetric, vertical cryostats. Compared to tension-loaded centerings, these have i.a. the advantage that no additional fixed anchoring on the second cryocontainer or the outer vacuum shell is required. This simplifies the construction.
The cryocontainers contract to a greater or lesser extent during cooling (depending on their final operating temperature), whereas the outer vacuum shell remains at room temperature and maintains its size. The position of the containers relative to each other and relative to the outer jacket can thereby change. Two containers could come into contact with another, in the present case most likely in the region of their inner tubes. This would disadvantageously lead to heat bridges and an increase in cryogenic losses. This is prevented through use of the centering elements. Since the centering elements generally also contract during cooling and since mutual adjustment of the containers is not possible in the cold state, the elements must be pretensioned in the warm state (for pressure centering), or the elements are loaded to a greater extent after cooling than in the warm state (for tension centering). This different, poorly defined tension state must be taken into consideration in the design of the elements.
These problems are aggravated when, in addition to the suspension tubes, asymmetrically disposed vertical openings (neck tubes) are required between the helium container or nitrogen container and the outer vacuum shell, as e.g. in vertical cryostats, when a cryocooler is used for recondensation of evaporating cryogens. In this case, the above-mentioned neck tube connects the outer vacuum shell of the cryostat to the cryocontainer (see e.g. US2002/0002830 for a helium container). When these openings are elastic in a vertical direction (e.g. a corrugated bellows), the forces generated by evacuation of the space between the outer vacuum shell and the components within the cryostat produce a torque that acts on the container. This torque must already be compensated for in the warm state by the centerings at the lower and upper edges of the cryocontainers. To also ensure centering of the containers in the cold state, the centering elements on one side of the container must be pretensioned even more in the warm state (pressure centering) or have an even greater tension in the cold state (tension centering) compared to containers without asymmetric opening. The eccentric pretension can cause the containers to contact each other, mainly in the region of the inner tubes. This increases the cryogenic loss during cooling and sometimes even results in a vacuum breakdown when O-rings freeze and loose their sealing properties.
It is therefore the underlying purpose of the present invention to propose a cryostat configuration, with which centering elements are inserted between the container to be centered and a surrounding container, preferably the outer vacuum shell, in such a manner that the containers are always centered relative to each other and the centering elements are not overloaded in any operating state.
It is therefore the underlying purpose of the present invention to propose a cryostat configuration which permits pressure centering even after cooling of the cryocontainers without overloading the centering elements in such a manner that the different cryocontainers or the cryocontainer and outer jacket do not contact each other.